This invention relates generally to a test stand and method for calibrating aircraft fuel components. More specifically, the present invention is directed to a test stand and method which utilizes a fuel look-alike with the properties of aircraft fuel, but without its low flash point.
During development, after initial assembly, and periodically throughout their lives, aircraft fuel components such as fuel pumps and fuel controls need to be evaluated and calibrated to ensure that they are performing within specified parameters of pressure, pressure drop and flow. Aircraft component manufacturers and post-depot overhaul facilities accomplish this calibration with test stands which simulate the operation of aircraft fuel systems. The components on test are mounted on the stand and fuel is passed through them. An operator sets input conditions, and adjusts the components until the desired outputs are reached.
Instead of using actual aircraft fuel, these test stands generally use a fuel "look-alike" which has the properties of aircraft fuel. MIL-F-7024 calibration fluid is normally the fluid of choice. It has a viscosity of 1.9 cs (at 25.degree. C.), a specific gravity of 0.76, and a dielectric constant of approximately 25-30 KV/cm2. Thus it is similar to JP4, which has a viscosity of 0.9 cs (at 100.degree. F.), specific gravity of 0.76 and a dielectric constant of 25-30 KV/cm2; and JP5, which has a viscosity of 1.4 cs (at 100.degree. F.), a specific gravity of 0.81 and a dielectric constant of 25-30 KV/cm2. Although MIL-F-7024 provides an accurate simulation, like most other fluids with similar properties it has a low flash point. In fact, its flash point of approximately 105.degree. F. makes it a hazardous material under the Department of Transportation definition, which includes liquids with flash points below 200.degree. F. This is also true for JP4 and JP5, with flash points of 135.degree. F. and 150.degree. F. respectively.
The use of low flash point fluids is disadvantageous because it results in unwanted combustion when electrical elements inadvertently ignite fuel vapors. This has been a major source of operator injury and fatality and damage to the test stands. One method of addressing this problem is to "explosion proof" electrical components in/on the test stand by enclosing them in heavy aluminum or steel housings which contain the explosion or fire, thereby keeping it from propagating. National Electrical Manufacturers Association (NEMA) Type 7 enclosures are employed pursuant to the National Electrical Code (NEC), Article 500, Class I, Group D, which pertains to hazardous environments. Motors and the associated motor starters and start/stop buttons, potentiometers, tachometers, and even background lighting, are all NEMA Type 7.
Explosion proofing is disadvantageous because it does not guarantee against damage control. Unwanted combustion still occurs which damages electrical equipment contained in the NEMA Type 7 enclosures. Furthermore, explosion proofing increases test stand cost. NEMA Type 7 enclosures are approximately double the cost of NEMA Type 1 components, which are used in normal (NEC Article 400) environments. Overall expense is increased by 30-50, which is quite significant insofar as these test stands cost hundreds of thousands of dollars.